PID cascade control problem.

Dear friends,

I am relatively new to using matlab. Currently, I am trying to implement a PD cascade control for the Ball and beam system, one for ball position and one for beam angle. I have added the 2 PID controllers from the simulink library with default values. Now, when I try to simulate this model, the output graphs are mirror image of what I expect for a step input.

But when I implement the controllers separately for ball position control and the beam angle control, it shows correct output.

Can anybody tell me, where I am going wrong? Thanks in advance.

[Arkadiy Turevskiy]

Can you provide access to your model? hard to say what is going on without seeing it. Also, in general using PID Controller block with default values is not going to provide good performance - gains need to be tuned . Arkadiy